Memory device with emulated characteristics

ABSTRACT

A memory subsystem is provided including an interface circuit adapted for communication with a system and a majority of address or control signals of a first number of memory circuits. The interface circuit includes emulation logic for emulating at least one memory circuit of a second number.

RELATED APPLICATION

This application is a continuation of commonly-assigned U.S. patentapplication Ser. No. 11/762,010 entitled “Memory Device with EmulatedCharacteristics” filed Jun. 12, 2007 by Rajan, et al., which, in turn,is a continuation-in-part of commonly-assigned U.S. patent applicationSer. No. 11/461,420 entitled “System and Method for Simulating aDifferent Number of Memory Circuits” filed Jul. 31, 2006 by Rajan, etal., which are incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to digital memory such as used incomputers, and more specifically to organization and design of memorymodules such as DIMMs.

2. Background Art

Digital memories are utilized in a wide variety of electronic systems,such as personal computers, workstations, servers, consumer electronics,printers, televisions, and so forth. Digital memories are manufacturedas monolithic integrated circuits (“ICs” or “chips”). Digital memoriescome in several types, such as dynamic random access memory (DRAM),static random access memory (SRAM), flash memory, electrically erasableprogrammable read only memory (EEPROM), and so forth.

In some systems, the memory chips are coupled directly into the systemsuch as by being soldered directly to the system's main motherboard. Inother systems, groups of memory chips are first coupled into memorymodules, such as dual in-line memory modules (DIMMs), which are in turncoupled into a system by means of slots, sockets, or other connectors.Some types of memory modules include not only the memory chipsthemselves, but also some additional logic which interfaces the memorychips to the system. This logic may perform a variety of low levelfunctions, such as buffering or latching signals between the chips andthe system, but it may also perform higher level functions, such astelling the system what are the characteristics of the memory chips.These characteristics may include, for example, memory capacity, speed,latency, interface protocol, and so forth.

Memory capacity requirements of such systems are increasing rapidly.However, other industry trends such as higher memory bus speeds, smallform factor machines, etc. are reducing the number of memory moduleslots, sockets, connectors, etc. that are available in such systems.There is, therefore, pressure for manufacturers to use large capacitymemory modules in such systems.

However, there is also an exponential relationship between a memorychip's capacity and its price. As a result, large capacity memorymodules may be cost prohibitive in some systems.

What is needed, then, is an effective way to make use of low cost memorychips in manufacturing high capacity memory modules.

SUMMARY

A memory subsystem is provided including an interface circuit adaptedfor communication with a system and a majority of address or controlsignals of a first number of memory circuits. The interface circuitincludes emulation logic for emulating at least one memory circuit of asecond number.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system coupled to multiple memory circuits and aninterface circuit according to one embodiment of this invention.

FIG. 2 shows a buffered stack of DRAM circuits each having a dedicateddata path from the buffer chip and sharing a single address, control,and clock bus.

FIG. 3 shows a buffered stack of DRAM circuits having two address,control, and clock busses and two data busses.

FIG. 4 shows a buffered stack of DRAM circuits having one address,control, and clock bus and two data busses.

FIG. 5 shows a buffered stack of DRAM circuits having one address,control, and clock bus and one data bus.

FIG. 6 shows a buffered stack of DRAM circuits in which the buffer chipis located in the middle of the stack of DRAM chips.

FIG. 7 is a flow chart showing one method of storing information.

FIG. 8 shows a high capacity DIMM using buffered stacks of DRAM chipsaccording to one embodiment of this invention.

FIG. 9 is a timing diagram showing one embodiment of how the buffer chipmakes a buffered stack of DRAM circuits appear to the system or memorycontroller to use longer column address strobe (CAS) latency DRAM chipsthan is actually used by the physical DRAM chips.

FIG. 10 shows a timing diagram showing the write data timing expected byDRAM in a buffered stack, in accordance with another embodiment of thisinvention.

FIG. 11 is a timing diagram showing how write control signals aredelayed by a buffer chip in accordance with another embodiment of thisinvention.

FIG. 12 is a timing diagram showing early write data from a memorycontroller or an advanced memory buffer (AMB) according to yet anotherembodiment of this invention.

FIG. 13 is a timing diagram showing address bus conflicts caused bydelayed write operations.

FIG. 14 is a timing diagram showing variable delay of an activateoperation through a buffer chip.

FIG. 15 is a timing diagram showing variable delay of a prechargeoperation through a buffer chip.

FIG. 16 shows a buffered stack of DRAM circuits and the buffer chipwhich presents them to the system as if they were a single, larger DRAMcircuit, in accordance with one embodiment of this invention.

FIG. 17 is a flow chart showing a method of refreshing a plurality ofmemory circuits, in accordance with one embodiment of this invention.

FIG. 18 shows a block diagram of another embodiment of the invention.

DETAILED DESCRIPTION

The invention will be understood more fully from the detaileddescription given below and from the accompanying drawings ofembodiments of the invention which, however, should not be taken tolimit the invention to the specific embodiments described, but are forexplanation and understanding only.

FIG. 1 illustrates a system 100 including a system device 106 coupled toan interface circuit 102, which is in turn coupled to a plurality ofphysical memory circuits 104A-N. The physical memory circuits may be anytype of memory circuits. In some embodiments, each physical memorycircuit is a separate memory chip. For example, each may be a DDR2 DRAM.In some embodiments, the memory circuits may be symmetrical, meaningeach has the same capacity, type, speed, etc., while in otherembodiments they may be asymmetrical. For ease of illustration only,three such memory circuits are shown, but actual embodiments may use anyplural number of memory circuits. As will be discussed below, the memorychips may optionally be coupled to a memory module (not shown), such asa DIMM.

The system device may be any type of system capable of requesting and/orinitiating a process that results in an access of the memory circuits.The system may include a memory controller (not shown) through which itaccesses the memory circuits.

The interface circuit may include any circuit or logic capable ofdirectly or indirectly communicating with the memory circuits, such as abuffer chip, advanced memory buffer (AMB) chip, etc. The interfacecircuit interfaces a plurality of signals 108 between the system deviceand the memory circuits. Such signals may include, for example, datasignals, address signals, control signals, clock signals, and so forth.In some embodiments, all of the signals communicated between the systemdevice and the memory circuits are communicated via the interfacecircuit. In other embodiments, some other signals 110 are communicateddirectly between the system device (or some component thereof, such as amemory controller, an AMB, or a register) and the memory circuits,without passing through the interface circuit. In some such embodiments,the majority of signals are communicated via the interface circuit, suchthat L>M.

As will be explained in greater detail below, the interface circuitpresents to the system device an interface to emulated memory deviceswhich differ in some aspect from the physical memory circuits which areactually present. For example, the interface circuit may tell the systemdevice that the number of emulated memory circuits is different than theactual number of physical memory circuits. The terms “emulating”,“emulated”, “emulation”, and the like will be used in this disclosure tosignify emulation, simulation, disguising, transforming, converting, andthe like, which results in at least one characteristic of the memorycircuits appearing to the system device to be different than the actual,physical characteristic. In some embodiments, the emulatedcharacteristic may be electrical in nature, physical in nature, logicalin nature, pertaining to a protocol, etc. An example of an emulatedelectrical characteristic might be a signal, or a voltage level. Anexample of an emulated physical characteristic might be a number of pinsor wires, a number of signals, or a memory capacity. An example of anemulated protocol characteristic might be a timing, or a specificprotocol such as DDR3.

In the case of an emulated signal, such signal may be a control signalsuch as an address signal, a data signal, or a control signal associatedwith an activate operation, precharge operation, write operation, moderegister read operation, refresh operation, etc. The interface circuitmay emulate the number of signals, type of signals, duration of signalassertion, and so forth. It may combine multiple signals to emulateanother signal.

The interface circuit may present to the system device an emulatedinterface to e.g. DDR3 memory, while the physical memory chips are, infact, DDR2 memory. The interface circuit may emulate an interface to oneversion of a protocol such as DDR2 with 5-5-5 latency timing, while thephysical memory chips are built to another version of the protocol suchas DDR2 with 3-3-3 latency timing. The interface circuit may emulate aninterface to a memory having a first capacity that is different than theactual combined capacity of the physical memory chips.

An emulated timing may relate to latency of e.g. a column address strobe(CAS) latency, a row address to column address latency (tRCD), a rowprecharge latency (tRP), an activate to precharge latency (tRAS), and soforth. CAS latency is related to the timing of accessing a column ofdata. tRCD is the latency required between the row address strobe (RAS)and CAS. tRP is the latency required to terminate an open row and openaccess to the next row. tRAS is the latency required to access a certainrow of data between an activate operation and a precharge operation.

The interface circuit may be operable to receive a signal from thesystem device and communicate the signal to one or more of the memorycircuits after a delay (which may be hidden from the system device).Such delay may be fixed, or in some embodiments it may be variable. Ifvariable, the delay may depend on e.g. a function of the current signalor a previous signal, a combination of signals, or the like. The delaymay include a cumulative delay associated with any one or more of thesignals. The delay may result in a time shift of the signal forward orbackward in time with respect to other signals. Different delays may beapplied to different signals. The interface circuit may similarly beoperable to receive a signal from a memory circuit and communicate thesignal to the system device after a delay.

The interface circuit may take the form of, or incorporate, or beincorporated into, a register, an AMB, a buffer, or the like, and maycomply with Joint Electron Device Engineering Council (JEDEC) standards,and may have forwarding, storing, and/or buffering capabilities.

In some embodiments, the interface circuit may perform operationswithout the system device's knowledge. One particularly useful suchoperation is a power-saving operation. The interface circuit mayidentify one or more of the memory circuits which are not currentlybeing accessed by the system device, and perform the power savingoperation on those. In one such embodiment, the identification mayinvolve determining whether any page (or other portion) of memory isbeing accessed. The power saving operation may be a power downoperation, such as a precharge power down operation.

The interface circuit may include one or more devices which togetherperform the emulation and related operations. The interface circuit maybe coupled or packaged with the memory devices, or with the systemdevice or a component thereof, or separately. In one embodiment, thememory circuits and the interface circuit are coupled to a DIMM.

FIG. 2 illustrates one embodiment of a system 200 including a systemdevice (e.g. host system 204, etc.) which communicates address, control,clock, and data signals with a memory subsystem 201 via an interface.

The memory subsystem includes a buffer chip 202 which presents the hostsystem with emulated interface to emulated memory, and a plurality ofphysical memory circuits which, in the example shown, are DRAM chips206A-D. In one embodiment, the DRAM chips are stacked, and the bufferchip is placed electrically between them and the host system. Althoughthe embodiments described here show the stack consisting of multipleDRAM circuits, a stack may refer to any collection of memory circuits(e.g. DRAM circuits, flash memory circuits, or combinations of memorycircuit technologies, etc.).

The buffer chip buffers communicates signals between the host system andthe DRAM chips, and presents to the host system an emulated interface topresent the memory as though it were a smaller number of larger capacityDRAM chips, although in actuality there is a larger number of smallercapacity DRAM chips in the memory subsystem. For example, there may beeight 512 Mb physical DRAM chips, but the buffer chip buffers andemulates them to appear as a single 4 Gb DRAM chip, or as two 2 Gb DRAMchips. Although the drawing shows four DRAM chips, this is for ease ofillustration only; the invention is, of course, not limited to usingfour DRAM chips.

In the example shown, the buffer chip is coupled to send address,control, and clock signals 208 to the DRAM chips via a single, sharedaddress, control, and clock bus, but each DRAM chip has its own,dedicated data path for sending and receiving data signals 210 to/fromthe buffer chip.

Throughout this disclosure, the reference number I will be used todenote the interface between the host system and the buffer chip, thereference number 2 will be used to denote the address, control, andclock interface between the buffer chip and the physical memorycircuits, and the reference number 3 will be used to denote the datainterface between the buffer chip and the physical memory circuits,regardless of the specifics of how any of those interfaces isimplemented in the various embodiments and configurations describedbelow. In the configuration shown in FIG. 2, there is a single address,control, and clock interface channel 2 and four data interface channels3; this implementation may thus be said to have a “1A4D” configuration(wherein “1A” means one address, control, and clock channel in interface2, and “4D” means four data channels in interface 3).

In the example shown, the DRAM chips are physically arranged on a singleside of the buffer chip. The buffer chip may, optionally, be a part ofthe stack of DRAM chips, and may optionally be the bottommost chip inthe stack. Or, it may be separate from the stack.

FIG. 3 illustrates another embodiment of a system 301 in which thebuffer chip 303 is interfaced to a host system 304 and is coupled to theDRAM chips 307A-307D somewhat differently than in the system of FIG. 2.There are a plurality of shared address, control, and clock busses 309Aand 309B, and a plurality of shared data busses 305A and 305B. Eachshared bus has two or more DRAM chips coupled to it. As shown, thesharing need not necessarily be the same in the data busses as it is inthe address, control, and clock busses. This embodiment has a “2A2D”configuration.

FIG. 4 illustrates another embodiment of a system 411 in which thebuffer chip 413 is interfaced to a host system 404 and is coupled to theDRAM chips 417A-417D somewhat differently than in the system of FIG. 2or 3. There is a shared address, control, and clock bus 419, and aplurality of shared data busses 415A and 415B. Each shared bus has twoor more DRAM chips coupled to it. This implementation has a “1A2D”configuration.

FIG. 5 illustrates another embodiment of a system 521 in which thebuffer chip 523 is interfaced to a host system 504 and is coupled to theDRAM chips 527A-527D somewhat differently than in the system of FIGS. 2through 4. There is a shared address, control, and clock bus 529, and ashared data bus 525. This implementation has a “1A1D” configuration.

FIG. 6 illustrates another embodiment of a system 631 in which thebuffer chip 633 is interfaced to a host system 604 and is coupled to theDRAM chips 637A-637D somewhat differently than in the system of FIGS. 2through 5. There is a plurality of shared address, control, and clockbusses 639A and 639B, and a plurality of dedicated data paths 635. Eachshared bus has two or more DRAM chips coupled to it. Further, in theexample shown, the DRAM chips are physically arranged on both sides ofthe buffer chip. There may be, for example, sixteen DRAM chips, with theeight DRAM chips on each side of the buffer chip arranged in two stacksof four chips each. This implementation has a “2A4D” configuration.

FIGS. 2 through 6 are not intended to be an exhaustive listing of allpossible permutations of data paths, busses, and buffer chipconfigurations, and are only illustrative of some ways in which the hostsystem device can be in electrical contact only with the load of thebuffer chip and thereby be isolated from whatever physical memorycircuits, data paths, busses, etc. exist on the (logical) other side ofthe buffer chip.

FIG. 7 illustrates one embodiment of a method 700 for storing at least aportion of information received in association with a first operation,for use in performing a second operation. Such a method may be practicedin a variety of systems, such as, but not limited to, those of FIGS.1-6. For example, the method may be performed by the interface circuitof FIG. 1 or the buffer chip of FIG. 2.

Initially, first information is received (702) in association with afirst operation to be performed on at least one of the memory circuits(DRAM chips). Depending on the particular implementation, the firstinformation may be received prior to, simultaneously with, or subsequentto the instigation of the first operation. The first operation may be,for example, a row operation, in which case the first information mayinclude e.g. address values received by the buffer chip via the addressbus from the host system. At least a portion of the first information isthen stored (704).

The buffer chip also receives (706) second information associated with asecond operation. For convenience, this receipt is shown as being afterthe storing of the first information, but it could also happen prior toor simultaneously with the storing. The second operation may be, forexample, a column operation.

Then, the buffer chip performs (708) the second operation, utilizing thestored portion of the first information, and the second information.

If the buffer chip is emulating a memory device which has a largercapacity than each of the physical DRAM chips in the stack, the bufferchip may receive from the host system's memory controller more addressbits than are required to address any given one of the DRAM chips. Inthis instance, the extra address bits may be decoded by the buffer chipto individually select the DRAM chips, utilizing separate chip selectsignals (not shown) to each of the DRAM chips in the stack.

For example, a stack of four ×4 1 Gb DRAM chips behind the buffer chipmay appear to the host system as a single ×4 4 Gb DRAM circuit, in whichcase the memory controller may provide sixteen row address bits andthree bank address bits during a row operation (e.g. an activateoperation), and provide eleven column address bits and three bankaddress bits during a column operation (e.g. a read or write operation).However, the individual DRAM chips in the stack may require onlyfourteen row address bits and three bank address bits for a rowoperation, and eleven column address bits and three bank address bitsduring a column operation. As a result, during a row operation (thefirst operation in the method 702), the buffer chip may receive twoaddress bits more than are needed by any of the DRAM chips. The bufferchip stores (704) these two extra bits during the row operation (inaddition to using them to select the correct one of the DRAM chips),then uses them later, during the column operation, to select the correctone of the DRAM chips.

The mapping between a system address (from the host system to the bufferchip) and a device address (from the buffer chip to a DRAM chip) may beperformed in various manners. In one embodiment, lower order system rowaddress and bank address bits may be mapped directly to the device rowaddress and bank address bits, with the most significant system rowaddress bits (and, optionally, the most significant bank address bits)being stored for use in the subsequent column operation. In one suchembodiment, what is stored is the decoded version of those bits; inother words, the extra bits may be stored either prior to or afterdecoding. The stored bits may be stored, for example, in an internallookup table (not shown) in the buffer chip, for one or more clockcycles.

As another example, the buffer chip may have four 512 Mb DRAM chips withwhich it emulates a single 2 Gb DRAM chip. The system will presentfifteen row address bits, from which the buffer chip may use thefourteen low order bits (or, optionally, some other set of fourteenbits) to directly address the DRAM chips. The system will present threebank address bits, from which the buffer chip may use the two low orderbits (or, optionally, some other set of two bits) to directly addressthe DRAM chips. During a row operation, the most significant bankaddress bit (or other unused bit) and the most significant row addressbit (or other unused bit) are used to generate the four DRAM chip selectsignals, and are stored for later reuse. And during a subsequent columnoperation, the stored bits are again used to generate the four DRAM chipselect signals. Optionally, the unused bank address is not stored duringthe row operation, as it will be re-presented during the subsequentcolumn operation.

As yet another example, addresses may be mapped between four 1 Gb DRAMcircuits to emulate a single 4 Gb DRAM circuit. Sixteen row address bitsand three bank address bits come from the host system, of which the loworder fourteen address bits and all three bank address bits are mappeddirectly to the DRAM circuits. During a row operation, the two mostsignificant row address bits are decoded to generate four chip selectsignals, and are stored using the bank address bits as the index. Duringthe subsequent column operation, the stored row address bits are againused to generate the four chip select signals.

A particular mapping technique may be chosen, to ensure that there areno unnecessary combinational logic circuits in the critical timing pathbetween the address input pins and address output pins of the bufferchip. Corresponding combinational logic circuits may instead be used togenerate the individual chip select signals. This may allow thecapacitive loading on the address outputs of the buffer chip to be muchhigher than the loading on the individual chip select signal outputs ofthe buffer chip.

In another embodiment, the address mapping may be performed by thebuffer chip using some of the bank address signals from the host systemto generate the chip select signals. The buffer chip may store thehigher order row address bits during a row operation, using the bankaddress as the index, and then use the stored address bits as part ofthe DRAM circuit bank address during a column operation.

For example, four 512 Mb DRAM chips may be used in emulating a single 2Gb DRAM. Fifteen row address bits come from the host system, of whichthe low order fourteen are mapped directly to the DRAM chips. Three bankaddress bits come from the host system, of which the least significantbit is used as a DRAM circuit bank address bit for the DRAM chips. Themost significant row address bit may be used as an additional DRAMcircuit bank address bit. During a row operation, the two mostsignificant bank address bits are decoded to generate the four chipselect signals. The most significant row address bit may be storedduring the row operation, and reused during the column operation withthe least significant bank address bit, to form the DRAM circuit bankaddress.

The column address from the host system memory controller may be mappeddirectly as the column address to the DRAM chips in the stack, sinceeach of the DRAM chips may have the same page size, regardless anydifferences in the capacities of the (asymmetrical) DRAM chips.

Optionally, address bit A(10) may be used by the memory controller toenable or disable auto-precharge during a column operation, in whichcase the buffer chip may forward that bit to the DRAM circuits withoutany modification during a column operation.

In various embodiments, it may be desirable to determine whether thesimulated DRAM circuit behaves according to a desired DRAM standard orother design specification. Behavior of many DRAM circuits is specifiedby the JEDEC standards, and it may be desirable to exactly emulate aparticular JEDEC standard DRAM. The JEDEC standard defines controlsignals that a DRAM circuit must accept and the behavior of the DRAMcircuit as a result of such control signals. For example, the JEDECspecification for DDR2 DRAM is known as JESD79-2B. If it is desired todetermine whether a standard is met, the following algorithm may beused. Using a set of software verification tools, it checks for formalverification of logic, that protocol behavior of the simulated DRAMcircuit is the same as the desired standard or other designspecification. Examples of suitable verification tools include:Magellan, supplied by Synopsys, Inc. of 700 E. Middlefield Rd., Mt.View, Calif. 94043; Incisive, supplied by Cadence Design Systems, Inc.,of 2655 Sealy Ave., San Jose, Calif. 95134; tools supplied by JasperDesign Automation, Inc. of 100 View St. #100, Mt. View, Calif. 94041;Verix, supplied by Real Intent, Inc., of 505 N. Mathilda Ave. #210,Sunnyvale, Calif. 94085; 0-In, supplied by Mentor Graphics Corp. of 8005SW Boeckman Rd., Wilsonville, Oreg. 97070; and others. These softwareverification tools use written assertions that correspond to the rulesestablished by the particular DRAM protocol and specification. Thesewritten assertions are further included in the code that forms the logicdescription for the buffer chip. By writing assertions that correspondto the desired behavior of the emulated DRAM circuit, a proof may beconstructed that determines whether the desired design requirements aremet.

For instance, an assertion may be written that no two DRAM controlsignals are allowed to be issued to an address, control, and clock busat the same time. Although one may know which of the various bufferchip/DRAM stack configurations and address mappings (such as thosedescribed above) are suitable, the verification process allows adesigner to prove that the emulated DRAM circuit exactly meets therequired standard etc. If, for example, an address mapping that uses acommon bus for data and a common bus for address, results in a controland clock bus that does not meet a required specification, alternativedesigns for buffer chips with other bus arrangements or alternativedesigns for the sideband signal interconnect between two or more bufferchips may be used and tested for compliance. Such sideband signalsconvey the power management signals, for example.

FIG. 8 illustrates a high capacity DIMM 800 using a plurality ofbuffered stacks of DRAM circuits 802 and a register device 804,according to one embodiment of this invention. The register performs theaddressing and control of the buffered stacks. In some embodiments, theDIMM may be an FB-DIMM, in which case the register is an AMB. In oneembodiment the emulation is performed at the DIMM level.

FIG. 9 is a timing diagram illustrating a timing design 900 of a bufferchip which makes a buffered stack of DRAM chips mimic a larger DRAMcircuit having longer CAS latency, in accordance with another embodimentof this invention. Any delay through a buffer chip may be madetransparent to the host system's memory controller, by using such amethod. Such a delay may be a result of the buffer chip being locatedelectrically between the memory bus of the host system and the stackedDRAM circuits, since some or all of the signals that connect the memorybus to the DRAM circuits pass through the buffer chip. A finite amountof time may be needed for these signals to traverse through the bufferchip. With the exception of register chips and AMBs, industry standardmemory protocols may not comprehend the buffer chip that sits betweenthe memory bus and the DRAM chips. Industry standards narrowly definethe properties of a register chip and an AMB, but not the properties ofthe buffer chip of this embodiment. Thus, any signal delay caused by thebuffer chip may cause a violation of the industry standard protocols.

In one embodiment, the buffer chip may cause a one-half clock cycledelay between the buffer chip receiving address and control signals fromthe host system memory controller (or, optionally, from a register chipor an AMB), and the address and control signals being valid at theinputs of the stacked DRAM circuits. Data signals may also have aone-half clock cycle delay in either direction to/from the host system.Other amounts of delay are, of course, possible, and the half-clockcycle example is for illustration only.

The cumulative delay through the buffer chip is the sum of a delay ofthe address and control signals and a delay of the data signals. FIG. 9illustrates an example where the buffer chip is using DRAM chips havinga native CAS latency of i clocks, and the buffer chip delay is j clocks,thus the buffer chip emulates a DRAM having a CAS latency of i+j clocks.In the example shown, the DRAM chips have a native CAS latency 906 offour clocks (from t1 to t5), and the total latency through the bufferchip is two clocks (one clock delay 902 from t0 to t1 for address andcontrol signals, plus one clock delay 904 from t5 to t6 for datasignals), and the buffer chip emulates a DRAM having a six clock CASlatency 908.

In FIG. 9 (and other timing diagrams), the reference numbers 1, 2,and/or 3 at the left margin indicate which of the interfaces correspondto the signals or values illustrated on the associated waveforms. Forexample, in FIG. 9: the “Clock” signal shown as a square wave on theuppermost waveform is indicated as belonging to the interface 1 betweenthe host system and the buffer chip; the “Control Input to Buffer”signal is also part of the interface 1; the “Control Input to DRAM”waveform is part of the interface 2 from the buffer chip to the physicalmemory circuits; the “Data Output from DRAM” waveform is part of theinterface 3 from the physical memory circuits to the buffer chip; andthe “Data Output from Buffer” shown in the lowermost waveform is part ofthe interface 1 from the buffer chip to the host system.

FIG. 10 is a timing diagram illustrating a timing design 1000 of writedata timing expected by a DRAM circuit in a buffered stack. Emulation ofa larger capacity DRAM circuit having higher CAS latency (as in FIG. 9)may, in some implementations, create a problem with the timing of writeoperations. For example, with respect to a buffered stack of DDR2 SDRAMchips with a read CAS latency of four clocks which are used in emulatinga single larger DDR2 SDRAM with a read CAS latency of six clocks, theDDR2 SDRAM protocol may specify that the write CAS latency 1002 is oneless than the read CAS latency. Therefore, since the buffered stackappears as a DDR2 SDRAM with a read CAS latency of six clocks, thememory controller may use a buffered stack write CAS latency of fiveclocks 1004 when scheduling a write operation to the memory.

In the specific example shown, the memory controller issues the writeoperation at t0. After a one clock cycle delay through the buffer chip,the write operation is issued to the DRAM chips at t1. Because thememory controller believes it is connected to memory having a read CASlatency of six clocks and thus a write CAS latency of five clocks, itissues the write data at time t0+5=t5. But because the physical DRAMchips have a read CAS latency of four clocks and thus a write CASlatency of three clocks, they expect to receive the write data at timet1+3=t4. Hence the problem, which the buffer chip may alleviate bydelaying write operations.

The waveform “Write Data Expected by DRAM” is not shown as belonging tointerface 1, interface 2, or interface 3, for the simple reason thatthere is no such signal present in any of those interfaces. Thatwaveform represents only what is expected by the DRAM, not what isactually provided to the DRAM.

FIG. 11 is a timing illustrating a timing design 1100 showing how thebuffer chip does this. The memory controller issues the write operationat t0. In FIG. 10, the write operation appeared at the DRAM circuits oneclock later at t1, due to the inherent delay through the buffer chip.But in FIG. 11, in addition to the inherent one clock delay, the bufferchip has added an extra two clocks of delay to the write operation,which is not issued to the DRAM chips until t0+1+2=t3. Because the DRAMchips receive the write operation at t3 and have a write CAS latency ofthree clocks, they expect to receive the write data at t3+3=t6. Becausethe memory controller issued the write operation at t0, and it expects awrite CAS latency of five clocks, it issues the write data at timet0+5=t5. After a one clock delay through the buffer chip, the write dataarrives at the DRAM chips at t5+1=t6, and the timing problem is solved.

It should be noted that extra delay of j clocks (beyond the inherentdelay) which the buffer chip deliberately adds before issuing the writeoperation to the DRAM is the sum j clocks of the inherent delay of theaddress and control signals and the inherent delay of the data signals.In the example shown, both those inherent delays are one clock, so j=2.

FIG. 12 is a timing diagram illustrating operation of an FB-DIMM's AMB,which may be designed to send write data earlier to buffered stacksinstead of delaying the write address and operation (as in FIG. 11).Specifically, it may use an early write CAS latency 1202 to compensatethe timing of the buffer chip write operation. If the buffer chip has acumulative (address and data) inherent delay of two clocks, the AMB maysend the write data to the buffered stack two clocks early. This may notbe possible in the case of registered DIMMs, in which the memorycontroller sends the write data directly to the buffered stacks (ratherthan via the AMB). In another embodiment, the memory controller itselfcould be designed to send write data early, to compensate for the jclocks of cumulative inherent delay caused by the buffer chip.

In the example shown, the memory controller issues the write operationat t0. After a one clock inherent delay through the buffer chip, thewrite operation arrives at the DRAM at t1. The DRAM expects the writedata at t1+3=t4. The industry specification would suggest a nominalwrite data time of t0+5=t5, but the AMB (or memory controller), whichalready has the write data (which are provided with the writeoperation), is configured to perform an early write at t5−2=t3. Afterthe inherent delay 1203 through the buffer chip, the write data arriveat the DRAM at t3+1=t4, exactly when the DRAM expects it—specifically,with a three-cycle DRAM Write CAS latency 1204 which is equal to thethree-cycle Early Write CAS Latency 1202.

FIG. 13 is a timing diagram 1300 illustrating bus conflicts which can becaused by delayed write operations. The delaying of write addresses andwrite operations may be performed by a buffer chip, a register, an AMB,etc. in a manner that is completely transparent to the memory controllerof the host system. And, because the memory controller is unaware ofthis delay, it may schedule subsequent operations such as activate orprecharge operations, which may collide with the delayed writes on theaddress bus to the DRAM chips in the stack.

An example is shown, in which the memory controller issues a writeoperation 1302 at time t0. The buffer chip or AMB delays the writeoperation, such that it appears on the bus to the DRAM chips at time t3.Unfortunately, at time t2 the memory controller issued an activateoperation (control signal) 1304 which, after a one-clock inherent delaythrough the buffer chip, appears on the bus to the DRAM chips at timet3, colliding with the delayed write.

FIGS. 14 and 15 are a timing diagram 1400 and a timing diagram 1500illustrating methods of avoiding such collisions. If the cumulativelatency through the buffer chip is two clock cycles, and the native readCAS latency of the DRAM chips is four clock cycles, then in order tohide the delay of the address and control signals and the data signalsthrough the buffer chip, the buffer chip presents the host system withan interface to an emulated memory having a read CAS latency of sixclock cycles. And if the tRCD and tRP of the DRAM chips are four clockcycles each, the buffer chip tells the host system that they are sixclock cycles each in order to allow the buffer chip to delay theactivate and precharge operations to avoid collisions in a manner thatis transparent to the host system.

For example, a buffered stack that uses 4-4-4 DRAM chips (that is, CASlatency=4, tRCD=4, and tRP=4) may appear to the host system as onelarger DRAM that uses 6-6-6 timing.

Since the buffered stack appears to the host system's memory controlleras having a tRCD of six clock cycles, the memory controller may schedulea column operation to a bank six clock cycles (at time t6) after anactivate (row) operation (at time t0) to the same bank. However, theDRAM chips in the stack actually have a tRCD of four clock cycles. Thisgives the buffer chip time to delay the activate operation by up to twoclock cycles, avoiding any conflicts on the address bus between thebuffer chip and the DRAM chips, while ensuring correct read and writetiming on the channel between the memory controller and the bufferedstack.

As shown, the buffer chip may issue the activate operation to the DRAMchips one, two, or three clock cycles after it receives the activateoperation from the memory controller, register, or AMB. The actual delayselected may depend on the presence or absence of other DRAM operationsthat may conflict with the activate operation, and may optionally changefrom one activate operation to another. In other words, the delay may bedynamic. A one-clock delay (1402A, 1502A) may be accomplished simply bythe inherent delay through the buffer chip. A two-clock delay (1402B,1502B) may be accomplished by adding one clock of additional delay tothe one-clock inherent delay, and a three-clock delay (1402C, 1502C) maybe accomplished by adding two clocks of additional delay to theone-clock inherent delay. A read, write, or activate operation issued bythe memory controller at time t6 will, after a one-clock inherent delaythrough the buffer chip, be issued to the DRAM chips at time t7. Apreceding activate or precharge operation issued by the memorycontroller at time t0 will, depending upon the delay, be issued to theDRAM chips at time t1, t2, or t3, each of which is at least the tRCD ortRP of four clocks earlier than the t7 issuance of the read, write, oractivate operation.

Since the buffered stack appears to the memory controller to have a tRPof six clock cycles, the memory controller may schedule a subsequentactivate (row) operation to a bank a minimum of six clock cycles afterissuing a precharge operation to that bank. However, since the DRAMcircuits in the stack actually have a tRP of four clock cycles, thebuffer chip may have the ability to delay issuing the prechargeoperation to the DRAM chips by up to two clock cycles, in order to avoidany conflicts on the address bus, or in order to satisfy the tRASrequirements of the DRAM chips.

In particular, if the activate operation to a bank was delayed to avoidan address bus conflict, then the precharge operation to the same bankmay be delayed by the buffer chip to satisfy the tRAS requirements ofthe DRAM. The buffer chip may issue the precharge operation to the DRAMchips one, two, or three clock cycles after it is received. The delayselected may depend on the presence or absence of address bus conflictsor tRAS violations, and may change from one precharge operation toanother.

FIG. 16 illustrates a buffered stack 1600 according to one embodiment ofthis invention. The buffered stack includes four 512 Mb DDR2 DRAMcircuits (chips) 1602 which a buffer chip 1604 maps to a single 2 GbDDR2 DRAM.

Although the multiple DRAM chips appear to the memory controller asthough they were a single, larger DRAM, the combined power dissipationof the actual DRAM chips may be much higher than the power dissipationof a monolithic DRAM of the same capacity. In other words, the physicalDRAM may consume significantly more power than would be consumed by theemulated DRAM.

As a result, a DIM M containing multiple buffered stacks may dissipatemuch more power than a standard DIMM of the same actual capacity usingmonolithic DRAM circuits. This increased power dissipation may limit thewidespread adoption of DIMMs that use buffered stacks. Thus, it isdesirable to have a power management technique which reduces the powerdissipation of DIMMs that use buffered stacks.

In one such technique, the DRAM circuits may be opportunistically placedin low power states or modes. For example, the DRAM circuits may beplaced in a precharge power down mode using the clock enable (CKE) pinof the DRAM circuits.

A single rank registered DIMM (R-DIMM) may contain a plurality ofbuffered stacks, each including four ×4 512Mb DDR2 SDRAM chips andappear (to the memory controller via emulation by the buffer chip) as asingle ×4 2 Gb DDR2 SDRAM. The JEDEC standard indicates that a 2 Gb DDR2SDRAM may generally have eight banks, shown in FIG. 16 as Bank 0 to Bank7. Therefore, the buffer chip may map each 512 Mb DRAM chip in the stackto two banks of the equivalent 2 Gb DRAM, as shown; the first DRAM chip1602A is treated as containing banks 0 and 1, 1602B is treated ascontaining banks 2 and 4, and so forth.

The memory controller may open and close pages in the DRAM banks basedon memory requests it receives from the rest of the host system. In someembodiments, no more than one page may be able to be open in a bank atany given time. In the embodiment shown in FIG. 16, each DRAM chip maytherefore have up to two pages open at a time. When a DRAM chip has noopen pages, the power management scheme may place it in the prechargepower down mode.

The clock enable inputs of the DRAM chips may be controlled by thebuffer chip, or by another chip (not shown) on the R-DIMM, or by an AMB(not shown) in the case of an FB-DIMM, or by the memory controller, toimplement the power management technique. The power management techniquemay be particularly effective if it implements a closed page policy.

Another optional power management technique may include mapping aplurality of DRAM circuits to a single bank of the larger capacityemulated DRAM. For example, a buffered stack (not shown) of sixteen ×4256 Mb DDR2 SDRAM chips may be used in emulating a single x4 4 Gb DDR2SDRAM. The 4 Gb DRAM is specified by JEDEC as having eight banks of 512Mbs each, so two of the 256 Mb DRAM chips may be mapped by the bufferchip to emulate each bank (whereas in FIG. 16 one DRAM was used toemulate two banks).

However, since only one page can be open in a bank at any given time,only one of the two DRAM chips emulating that bank can be in the activestate at any given time. If the memory controller opens a page in one ofthe two DRAM chips, the other may be placed in the precharge power downmode. Thus, if a number p of DRAM chips are used to emulate one bank, atleast p−1 of them may be in a power down mode at any given time; inother words, at least p−1 of the p chips are always in power down mode,although the particular powered down chips will tend to change overtime, as the memory controller opens and closes various pages of memory.

As a caveat on the term “always” in the preceding paragraph, the powersaving operation may comprise operating in precharge power down modeexcept when refresh is required.

FIG. 17 is a flow chart 1700 illustrating one embodiment of a method ofrefreshing a plurality of memory circuits. A refresh control signal isreceived (1702) e.g. from a memory controller which intends to refreshan emulated memory circuit. In response to receipt of the refreshcontrol signal, a plurality of refresh control signals are sent (1704)e.g. by a buffer chip to a plurality of physical memory circuits atdifferent times. These refresh control signals may optionally includethe received refresh control signal or an instantiation or copy thereof.They may also, or instead, include refresh control signals that aredifferent in at least one aspect (format, content, etc.) from thereceived signal.

In some embodiments, at least one first refresh control signal may besent to a first subset of the physical memory circuits at a first time,and at least one second refresh control signal may be sent to a secondsubset of the physical memory circuits at a second time. Each refreshsignal may be sent to one physical memory circuit, or to a plurality ofphysical memory circuits, depending upon the particular implementation.

The refresh control signals may be sent to the physical memory circuitsafter a delay in accordance with a particular timing. For example, thetiming in which they are sent to the physical memory circuits may beselected to minimize an electrical current drawn by die memory, or tominimize a power consumption of the memory. This may be accomplished bystaggering a plurality of refresh control signals. Or, the timing may beselected to comply with e.g. a tRFC parameter associated with the memorycircuits.

To this end, physical DRAM circuits may receive periodic refreshoperations to maintain integrity of data stored therein. A memorycontroller may initiate refresh operations by issuing refresh controlsignals to the DRAM circuits with sufficient frequency to prevent anyloss of data in the DRAM circuits. After a refresh control signal isissued, a minimum time tRFC may be required to elapse before anothercontrol signal may be issued to that DRAM circuit. The tRFC parametervalue may increase as the size of the DRAM circuit increases.

When the buffer chip receives a refresh control signal from the memorycontroller, it may refresh the smaller DRAM circuits within the span oftime specified by the tRFC of the emulated DRAM circuit. Since the IRFCof the larger, emulated DRAM is longer than the tRFC of the smaller,physical DRAM circuits, it may not be necessary to issue any or all ofthe refresh control signals to the physical DRAM circuitssimultaneously. Refresh control signals may be issued separately toindividual DRAM circuits or to groups of DRAM circuits, provided thatthe tRFC requirements of all physical DRAMs has been met by the time theemulated DRAM's tRFC has elapsed. In use, the refreshes may be spaced intime to minimize the peak current draw of the combination buffer chipand DRAM circuit set during a refresh operation.

FIG. 18 illustrates one embodiment of an interface circuit such as maybe utilized in any of the above-described memory systems, forinterfacing between a system and memory circuits. The interface circuitmay be included in the buffer chip, for example.

The interface circuit includes a system address signal interface forsending/receiving address signals to/from the host system, a systemcontrol signal interface for sending/receiving control signals to/fromthe host system, a system clock signal interface for sending/receivingclock signals to/from the host system, and a system data signalinterface for sending/receiving data signals to/from the host system.The interface circuit further includes a memory address signal interfacefor sending/receiving address signals to/from the physical memory, amemory control signal interface for sending/receiving control signalsto/from the physical memory, a memory clock signal interface forsending/receiving clock signals to/from the physical memory, and amemory data signal interface for sending/receiving data signals to/fromthe physical memory.

The host system includes a set of memory attribute expectations, orbuilt-in parameters of the physical memory with which it has beendesigned to work (or with which it has been told, e.g. by the buffercircuit, it is working). Accordingly, the host system includes a set ofmemory interaction attributes, or built-in parameters according to whichthe host system has been designed to operate in its interactions withthe memory. These memory interaction attributes and expectations willtypically, but not necessarily, be embodied in the host system's memorycontroller.

In addition to physical storage circuits or devices, the physical memoryitself has a set of physical attributes.

These expectations and attributes may include, by way of example only,memory timing, memory capacity, memory latency, memory functionality,memory type, memory protocol, memory power consumption, memory currentrequirements, and so forth.

The interface circuit includes memory physical attribute storage forstoring values or parameters of various physical attributes of thephysical memory circuits. The interface circuit further includes systememulated attribute storage. These storage systems may be read/writecapable stores, or they may simply be a set of hard-wired logic orvalues, or they may simply be inherent in the operation of the interfacecircuit.

The interface circuit includes emulation logic which operates accordingto the stored memory physical attributes and the stored system emulationattributes, to present to the system an interface to an emulated memorywhich differs in at least one attribute from the actual physical memory.The emulation logic may, in various embodiments, alter a timing, value,latency, etc. of any of the address, control, clock, and/or data signalsit sends to or receives from the system and/or the physical memory. Somesuch signals may pass through unaltered, while others may be altered.The emulation logic may be embodied as, for example, hard wired logic, astate machine, software executing on a processor, and so forth.

CONCLUSION

When one component is said to be “adjacent” another component, it shouldnot be interpreted to mean that there is absolutely nothing between thetwo components, only that they are in the order indicated.

The physical memory circuits employed in practicing this invention maybe any type of memory whatsoever, such as: DRAM, DDR DRAM, DDR2 DRAM,DDR3 DRAM, SDRAM, QDR DRAM, DRDRAM, FPM DRAM, VDRAMI, EDO DRAM, BEDODRAM, MDRAM, SGRAM, MRAM, IRAM, NAND flash, NOR flash, PSRAM, wetwarememory, etc.

The physical memory circuits may be coupled to any type of memorymodule, such as: DIMM, R-DIMM, SO-DIMM, FB-DIMM, unbuffered DIMM, etc.

The system device which accesses the memory may be any type of systemdevice, such as: desktop computer, laptop computer, workstation, server,consumer electronic device, television, personal digital assistant(PDA), mobile phone, printer or other peripheral device, etc.

The various features illustrated in the figures may be combined in manyways, and should not be interpreted as though limited to the specificembodiments in which they were explained and shown.

Those skilled in the art, having the benefit of this disclosure, willappreciate that many other variations from the foregoing description anddrawings may be made within the scope of the present invention. Indeed,the invention is not limited to the details described above. Rather, itis the following claims including any amendments thereto that define thescope of the invention.

1. A sub-system, comprising: an interface circuit adapted forcommunication with a system and a majority of address or control signalsof a first number of memory circuits, the interface circuit includingemulation logic for emulating at least one memory circuit of a secondnumber.
 2. The sub-system of claim 1, wherein the second number is lessthan the first number.
 3. The sub-system of claim 2, wherein the secondnumber is one.
 4. The sub-system of claim 1, wherein the emulation logicemulates at least one memory circuit with a first memory capacity thatis different than a second memory capacity of at least one of theplurality of memory circuits.
 5. The sub-system of claim 1, wherein theinterface circuit is adapted for communication with all of the addressor control signals of the memory circuits.
 6. The sub-system of claim 1,wherein the interface circuit is adapted for communication with amajority of the address signals of the memory circuits.
 7. Thesub-system of claim 1, wherein the interface circuit is adapted forcommunication with a majority of the control signals of the memorycircuits.
 8. The sub-system of claim 1, wherein the emulation includesan electrical emulation.
 9. The sub-system of claim 1, wherein theemulation includes a logical emulation.
 10. The sub-system of claim 1,wherein the interface circuit includes a buffer chip.
 11. The sub-systemof claim 1, wherein the interface circuit is positioned on a dualin-line memory module (DIMM).
 12. The sub-system of claim 11, whereinthe DIMM includes a small outline-DIMM (SO-DIMM).
 13. The sub-system ofclaim 11, wherein the DIMM includes a fully buffered-DIMM (FB-DIMM). 14.The sub-system of claim 11, wherein the DIMM includes a registered-DIMM(R-DIMM).
 15. The sub-system of claim 1, wherein the memory circuitseach include dynamic random access memory (DRAM).
 16. The sub-system ofclaim 15, wherein the memory circuits each include a monolithic DRAM.17. The sub-system of claim 16, wherein the memory circuits are stacked.18. The sub-system of claim 16, wherein the memory circuits and theinterface circuit are stacked.
 19. A method, comprising: interfacing amajority of address or control signals of a first number of memorycircuits and a system; and emulating at least one memory circuit of asecond number.
 20. An apparatus, comprising: a first number of memorycircuits; and an interface circuit in communication with the memorycircuits, the interface circuit including emulation logic for emulatingat least one memory circuit of a second number; wherein the interfacecircuit interfaces a majority of address or control signals of thememory circuits.